1. Field of the Invention
This invention relates to wireless communication technologies and, more particularly, to an apparatus and method for reducing radiated electromagnetic coupling between two or more antennas coupled to a wireless communications device.
2. Description of the Related Art
The following descriptions and examples are not admitted to be prior art by virtue of their inclusion within this section.
The term “wireless” is often used to describe telecommunications in which electromagnetic waves (rather than some form of wire) carry a transmitted signal over part or all of the communication path. Some monitoring devices, such as intrusion alarms, employ acoustic waves at frequencies above the range of human hearing; these are also sometimes classified as wireless.
As wireless communication technologies continue to evolve and mature, a growing number of wireless communication devices are becoming available. Examples of wireless communication devices in use today include: cellular telephones (“cell phones”) and pagers, Personal Digital Assistants (PDA), Global Positioning Systems (GPS), cordless computer peripherals (e.g., cordless keyboards, printers and mice), cordless telephones (i.e., limited-range devices, not to be confused with cell phones), home-entertainment-system control boxes (e.g., VCR and TV channel control boxes, in addition to some hi-fi sound systems and FM broadcast receivers), remote garage-door openers, two-way radios, and satellite TV and radio devices. These communication devices, and numerous others, typically communicate over a variety of networks including: wireless local area networks (WLAN), wireless wide area networks (WWAN) and wireless personal area networks (WPAN).
Wireless Local Area Network (WLAN) technologies allow communication to a Local Area Network (or LAN, such as an Ethernet or Token Ring network), which may reside within a building, on a campus, or in public “hotspot” areas such as hotels or airports. Numerous specifications and protocols have been defined for accessing WLANs over relatively short distances (i.e., up to about 100 meters). The Institute of Electrical and Electronics Engineers (IEEE), for example, has defined a family of specifications (i.e., IEEE 802.11, 802.11a, 802.11b, and 802.11g standards) for controlling access to WLANs. Other specifications for accessing a WLAN may include: HomeRF™, OpenAir™, HiperLAN1, and HiperLAN2. The term “WLAN” is sometimes used to refer to a class of wireless communication technologies that operates over a distance of up to 100 meters.
Wireless Wide Area Network (WWAN) technologies allow communication to a geographically dispersed Wide Area Network (WAN). WWAN technologies use various devices (e.g., telephone lines, satellite dishes, and radio waves) to service an area broader than that which can be covered by a WLAN. WWAN technologies are typically used in cellular telecommunications and may include, for example: Global System for Mobile communication (GSM), a digital cell phone service widely used in Europe and other parts of the world; Personal Communications Services (PCS), a digital cell phone service widely used in the United States; Enhanced Data GSM Environment (EDGE), a faster version of the GSM service providing broadband access to mobile phones and computers; General Packet Radio Services (GPRS), a packet-based wireless communication service providing broadband access to mobile phones and computers; and Universal Mobile Telecommunications Service (UMTS), a packet-based wireless communication service providing a consistent set of broadband services to mobile phone and computer users, no matter where they are located in the world.
Wireless personal area network (WPAN) technologies allow communication to a Personal Area Network (PAN), or a network for interconnecting wireless devices that are centered around an individual person. Because WPAN technologies limit communications to a very short range (e.g., about 10 meters), they are specifically targeted as cable replacement wireless technologies for a range of diverse computing and telecommunication devices, such as portable PCs (e.g., notebook and tablet computers), peripherals (e.g., cordless keyboards, printers and mice), handheld devices (e.g., PDAs), cell phones, pagers and other consumer electronics. As such, a WPAN may serve to interconnect some or all of the computing and telecommunication devices that many people use in their home or office on a daily basis. The term “WPAN” may also be used to refer to a class of wireless communication technologies that operates over a distance of up to 10 meters.
Though technology for WPAN is in its infancy, compared to WLAN and WWAN technologies, it is undergoing rapid development. A key concept in WPAN technologies is known as “plugging in,” which allows any two WPAN-equipped devices to communicate—as if they were connected by a cable—when the devices come within close proximity to each other (e.g., within several meters) or a central server (e.g., within a few kilometers). A primary objective of WPAN technology is to facilitate seamless operation among home and/or business WPAN-equipped devices and systems. Such an objective will allow every device in a particular WPAN to plug into any other device in the same WPAN, provided they are within an appropriate range of one another. Current WPAN technologies are addressed by the IEEE 802.15 standards, the initial version of which (IEEE 802.15.1) was adapted from a well-known and widely used specification known as “Bluetooth™.”
To allow efficient use of the radio frequency (RF) spectrum (or “radio spectrum”), various frequency bands within the 3 KHz to 300 GHz RF spectrum are allocated to certain types of wireless communications. The allocated bands are then divided into a number of “channels” of equal bandwidth, so that a number of wireless devices may share the same band. However, problems often arise when two radios, operating within the same or different wireless device, come within a relatively close proximity of one another. For example, “interference” occurs when electromagnetic energy from a transmitted signal is coupled into the receive path of a receiver within a nearby radio. In other words, “interference” occurs whenever a radio receives, in addition to a desired signal, the signals from other radios that may be transmitting within the same (or a nearby) radio frequency band. Since the RF spectrum is a limited resource and needs to be shared by as many users as possible, interference problems severely limit the performance of many wireless communication devices by decreasing the dynamic range of the receiver, decreasing the throughput of the receiver, and in some cases, jamming radio operation altogether.
In an effort to address the interference problem, many wireless communications devices use spread-spectrum technologies to deliberately vary the transmission frequency, or channel, over which a signal is transmitted within a particular radio frequency (RF) band. The variation is done according to a specific, though complicated mathematical function that changes the transmission frequency abruptly, often many times a second. Most spread-spectrum technologies use a digital variation scheme called “frequency hopping,” where a relatively stable transmission frequency is maintained between “hops” for a length of time known as the “dwell time.” A few spread-spectrum technologies employ analog schemes to provide continuous frequency variation.
Unfortunately, frequency hopping alone cannot reduce the interference to an acceptable level when two or more radios, operating at the same time and/or on the same (or nearby) channel, come within close proximity to one another. As used herein, the term “close proximity” refers to a distance, which separates two or more co-located radios and results in an unacceptable level of interference when the radios operate at the same time and/or on the same channel. Unacceptable levels of interference are typically defined by the communication protocols associated with the interfering radios. The term “co-located” is used herein to describe two or more radios that are located within the same wireless communication device, or alternatively, within different devices in close proximity to one another.
Some wireless device manufacturers have chosen to avoid interference through strict adherence to the communication protocols associated with certain wireless communication technologies. For example, some manufacturers will spatially separate co-located radios by a sufficient distance (relative to the wavelength of transmitted signals) to achieve an appropriate amount of attenuation/suppression of the transmit signal in the receive path. Since the impact of interference on system performance is primarily related to the power of the desired signal (or carrier signal) relative to that of the interfering signal, most communication protocols specify the spatial separation in terms of a carrier-to-interference level (C/I) ratio. The C/I ratio is measured in units of decibels (dB) and may be approximately +11 dB to −40 dB for some communication protocols. Depending on the wavelength of the transmit signal, the C/I ratio desired by some communication protocols may require co-located radios to be separated by as much as eight feet or more. Clearly, such a large separation between radio modules is simply not possible within most portable computing and telecommunication devices (e.g., portable PCs, PDAs, cell phones, etc.), due to the relatively small size of these devices.
Other manufacturers choose to avoid interference altogether by permitting only one radio module to operate at any given time. This technique, however, tends to place unnecessary limitations on the user's ability to run multiple applications at the same time such as, e.g., downloading and displaying information on a PDA display screen while discussing the downloaded information with a colleague using a hands-free connection to the PDA. Though once considered a luxury, simultaneous radio operation is becoming a highly desirable feature for today's consumers, and therefore, a necessity for manufactures who want to maintain a competitive edge in a fast-paced market.
A few manufactures have attempted to reduce the level of interference between radio modules located within portable computing and telecommunication devices, while enabling the radios to operate at substantially the same time. Unfortunately, these manufacturers generally do so by adding intelligence (in the form of hardware and/or software) to the radio modules so that they will “sense” each other and avoid interference-causing collisions as much as possible. In some cases, intelligence is added to filter or otherwise subtract the interference signal from the desired signal. These additions are not only complicated, but also tend to consume valuable power resources (a limited commodity and weight consideration) in most portable computing and telecommunication devices. Adding intelligence may also lead to undesirable increases in the size, weight and cost of the wireless communication device, sometimes without reducing interference to the levels accepted by some communication protocols.
Other manufacturers may attempt to reduce interference by manipulating the radiation pattern(s) of the antenna(s) within co-located radio modules. For example, an antenna within an interfering radio module may be designed with directional properties to reduce interference by directing a substantial portion of the electromagnetic radiation away from a co-located radio in module. Unfortunately, the use of directional antennas (often referred to as “antenna arrays” or “beam steerers”) tends to increase the size, weight, cost, complexity and power consumption of the radio module, sometimes without reducing the interference to an “acceptable” level. Large financial investments and lengthy learning curves may also be involved if a new antenna design is to be engineered for a specific combination or arrangement of radios within a particular wireless communication device. In the fast-paced world of telecommunications, lengthy development times may cause manufactures to miss narrow market windows, causing them to forfeit profits and/or market share to competitors.
Therefore, it would be beneficial to provide a improved means for reducing electromagnetic interference between two or more co-located radio modules operating at the same time and in the same (or nearby) frequency band. Such means would preferably provide a low-cost, simple solution to the interference problem created within wireless communication devices, while conserving the power efficiency of a transmitting radio module, maintaining the integrity of a transmitted signal, and minimizing the size and weight of the wireless communication device.